1. Field of the Invention
The present invention relates to a disk drive that performs seek control for moving a head to a target position on a disk rotated by a spindle motor and track following control for settling the head within a target range for the target position. In particular, the present invention relates to an apparatus and method for calculating a disk shift amount in a disk drive which apparatus and method are suitable for calculating the amount of disk shift resulting from an external impact while the disk drive is inactive.
2. Description of the Related Art
Hard disk drives are well known as using a disk as a recording medium. The hard disk drive generally comprises a head positioning control system (servo system) that performs control for positioning a head at a target position on the disk. This control system handles an actuator in which the head is mounted, as a to-be-controlled object (that is, a plant). The actuator supports the head such that the head is movable in a radial direction of the disk. The control system controls driving of the actuator on the basis of servo information written (embedded) in the disk. The control system performs seek control for a seek operation of moving the head to a target track and track following control for a track following operation (tracking operation) of settling the head moved to the target track within a target range for the target track.
A disk runout that may occur as the disk rotates is known as a factor degrading the track following performance of the hard disk drive. The disk runout may result from, for example, an assembly error in the hard disk drive. The term “assembly error” as used in the specification refers to an error that may occur when the disk is screwed onto a hub of a spindle motor during a process for manufacturing the hard disk drive. With the track following control, the disk runout appears as the runout of a track. Accordingly, the head must be allowed to follow the runout. The component of the disk runout (runout component) is contained in a position error that may occur when the head is allowed to follow the target track. Thus, Jpn. Pat. Appln. KOKAI Publication No. 11-39814 (Document 1) discloses a technique for detecting the runout component of the disk runout that may occur in synchronism with the rotation of the disk to eliminate the runout component from the position error. The technique described in Document 1 can suppress the runout component for the track, which is in synchronism with the rotation of the disk, to reduce a tracking error that may occur in the tracking by the head.
The runout component is detected by adaptive learning. An initial value is required to carry out the adaptive learning in a short time. Thus, Jpn. Pat. Appln. KOKAI Publication No. 2002-230928 (Document 2) discloses learning of the runout component for each pre-specified radial position on the disk during the manufacture of the hard disk drive. Document 2 further discloses a technique for using, as an initial value for the adaptive learning, the runout component for each pre-specified radial position on the disk obtained through the learning during the manufacture of the hard disk drive (this runout component is called a learned value). The initial value for the adaptive learning is stored in a nonvolatile memory. Document 2 takes a temporal variation in the runout component for each radial position into consideration. Document 2 thus proposes that the learned value for the runout component for each radial position, which is used as an initial value for the adaptive learning, be corrected when the hard disk drive is activated. In this case, the runout component for a particular radial position of the disk is relearned. Then, the learned value (initial value) for the runout component for each radial position of the disk is corrected utilizing the rate of a change from the original learned value to the relearned value (or the difference between the original learned value and the relearned value). With the technique described in Document 2 (prior art), the head can be caused to precisely follow the target track without being affected by the runout of a track in synchronism with the rotation of the disk or by a temporal variation in the runout component.
When the hard disk drives incurs an excessive external impact, the center of the disk in the disk drive may be shifted from the rotating shaft of the spindle motor, that is, what is called a disk shift may occur. In this case, a very long time is required for the convergence of the learning of the runout component, which is carried out during the activation of the hard disk drive. In other words, the time required to activate the disk drive increases. However, the above prior art cannot determine whether the increase in activation time results from a disk shift or another factor. For example, the prior art cannot determine whether or not the increase in activation time results from a disk shift caused by an impact made (for example, the impact made when the disk drive falls) while the hard disk drive is inactive, the impact exceeding a value specified for the product. The factor mentioned above includes vibration externally applied to the disk drive or a defect in the actuator, which supports the head such that the head is movable.